1. Field of the Invention
The invention relates to a hydromechanical transmission with a hydraulic pump-driven hydraulic motor of a hydrostatic continuously variable transmission and a mechanical multi-step variable speed transmission connected to a drive shaft of the hydraulic motor, which can be shifted into operation by a gear shifting device.
2. Technical Considerations
A generic hydromechanical transmission is described in EP 1 231 413 A2 and can be used in mobile work machines, such as telehandlers and also construction and harvesting machinery. Owing to the two-step configuration of the multi-step variable speed transmission, it is possible to shift to and from a working gear and a road gear and, in contrast to so-called geared-neutral transmissions, it is possible to do so in operation, i.e., while the work machine is being driven. In the road gear, driving speeds of 40 km/hr and more are achievable for reaching a worksite of the work machine as quickly as possible. In the working gear of the multi-step variable speed transmission, i.e., at slow driving speeds of a work machine equipped with the hydromechanical transmission, as a general rule the continuously variable gear ratio range of the hydrostatic continuously variable transmission (CVT) suffices and does not require any additional gear ratio spread.
In order to be able to shift in operation, i.e., while driving, with a hydromechanical transmission with switching gear teeth in the multi-step variable speed transmission, provision is made of electrohydraulic synchronization. Changing the gear ratio in the CVT brings about a rotation speed adjustment corresponding to the step change in the multi-step variable speed transmission. Although the traction force in the multi-step variable speed transmission is briefly interrupted during the shifting operation, this has no negative impact during a road drive.
If the multi-step variable speed transmission is configured as a power shift transmission, then multi-plate clutches ensure shifting to and from the working gear and the road gear without interruption of the traction force. The shifting operation in such a multi-step variable speed transmission can be further assisted by shifting the gear ratio in the CVT toward a synchronization speed.
In the hydromechanical transmission of EP 1 231 413 A2, which is configured as a spur gear train and equipped with switching gear teeth, use is made of a hydraulic motor configured as a bent-axis motor, which is inserted in a transmission case of the multi-step variable speed transmission. An input shaft of the multi-step variable speed transmission engages with the gear teeth of a drive shaft of the hydraulic motor. The transmission input shaft is supported in a transmission case on one end and on the drive shaft of the hydraulic motor on the other. The gear shifting device of the multi-step variable speed transmission is associated with the multi-step variable speed transmission. A shift sleeve with an actuator lever and a hydraulic shift cylinder with electromagnetically actuable shift valves thus constitute integral components of the multi-step variable speed transmission.
As the CVT and the multi-step variable speed transmission in hydromechanical transmissions are often made by different manufacturers, extensive engineering measures are required in order to adapt the performance of the hydrostatic CVT and the shifting forces and shifting travels in the multi-step variable speed transmission to one another so that, given the characteristics and operating performance of the CVT, trouble-free shifting in the sense of an optimal shifting operation in the multi-step variable speed transmission is possible.
A change of manufacturers of the multi-step variable speed transmission entails an additional workload for the manufacturer of the CVT (who as a general rule is responsible for the entire system), because the gear shifting device operatively connected to the CVT is generally different from the previous one. It is therefore necessary to provide a gear shifting device suitably adapted to the CVT for each transmission combination, which disadvantageously leads to a high level of variance. Furthermore, this variance limits the design freedom in terms of optimizing the dimensions of the hydromechanical transmission.
Therefore it is an object of the present invention to provide a hydromechanical transmission of the generally aforesaid type but with a compact design that is readily adaptable to diverse installation cases.